This invention relates to a spray-drying granulation apparatus mainly constituted by a chamber which incorporates a granulation section.
In general, a powder obtained by spray drying is very fine and has an average particle size of about several tens to several hundreds of microns (e.g., 30 to 120 .mu.m). If such a powder is directly handled, it tends to be raised like dust or become undissolved lumps when dissolved in water,and has a low fluidity. To cope with these defects, the process of forming such powder is in many cases designed to be followed by a granulation process in which fine powder obtained by spray drying is aggregated (the average particle size after granulation is, in general, 3 to 6 times larger than that before the granulation).
The particle size of granules is changed depending upon the use of the granule product For instance, a creamy powder is used as, so to say, fine granules of a particle size of 400 to 800 .mu.m, a raw material of a herb medicine is used as granules of a particle size of 150 to 400 .mu.m from which tablets are formed, and an antibiotic needs to be granulated to have a particle size of 80 to 200 .mu.m.
Regarding to granulation of powder, a type of apparatus for successively forming granules is known in which a binder liquid is sprayed to powder which is suspending in an upward flow of a fluid (chiefly, air) and granules are successively formed by aggregation effected by the binder. This method utilizes aggregating phenomena due to the moisture in powder in order to form granules. It is said that the water content of powder ranges from 5 to 15% in the case of granulation of a powder such as powdered milk where the powder is allowed to have a water content varying over a comparatively wide range, and that it ranges from 5 to 8% in the case of an amino-acid containing material such as soy sauce or a solution of various extracts. However, in an actual apparatus, the progress of granulation is slow if the moisture (water content) of the powder is low, or the powder attaches to the apparatus or becomes lumps like dumpling if it is high. Thus, the suitable water content range is very narrow. For example, it is several percent in the case of powdered milk, and smaller than 1% in the case of an amino-acid containing material. Therefore, highly improved skill is needed to control the water content.
A combination of a spray drying apparatus and a fluidizing-granulation apparatus can be used to constitute a system for forming granules from liquid raw-materials. The following two types of apparatus have been proposed on the basis of this concept, though they leave some problems.
FIG. 5 shows a spray drying apparatus which is mainly constituted by a vessel which incorporates a fluidized-bed section. In FIG. 5 is illustrated a nozzle 100, a spray-drying section 101, a fluidizing-granulation section 102, drying hot air inlet 103, fluidized-bed-air introducing section 104, a first cyclone 105, a second cyclone 106, a product discharge vessel 107, and a feed liquid 108. In this well-known apparatus, the nozzle 100 for spray drying also serves as a nozzle for spraying a binder liquid for fluidization granulation, and the water content of the dried materials cannot be adequately reduced during spray drying. This is because granulation cannot be effected if the water content of the materials is lower than a certain level.
For this reason, the selection of factors including the relationship between the dispositions of the spray drying section 101 and the fluidizing-granulation section 102, the spraying angle of the nozzle, the viscosity and the density of the liquid to be sprayed, the diameter of liquid droplets, the temperatures and the flow rates of hot air and fluidization air, and so forth is very complicated, the range of suitable setting is very narrow.
The dried materials after spray drying are in an incompletely dried state and tends to attach the inner wall of the vessel. There is therefore a possibility of the attached materials being heat-denatured or a possibility of the operation becoming impossible by an increase of the amount of attached materials.
The degree of granulation is entirely determined by the drying conditions, and it is impossible to desirably change the size of granules.
An apparatus, such as that shown in FIG. 6, having a spray drying section X, powder collecting section Y and a fluidizing-granulation section F with binder feeding nozzle 201 provided in separate chambers has also been put into use. However, this apparatus requires a high construction cost as well as a high running cost. A reference character O in FIG. 6 denotes a product outlet.
In the spray drying section, it is necessary to adequately dry particles in order to supply them to the succeeding section and, in general, necessary to reduce the temperature or moisture of the exhaust. It is thereby desired to supply secondary air Z into the chamber and provide additional equipment of a large capacity including a blower and a wet scrubber used as after treatment equipment for the supply of secondary air.
The fluidizing-granulation section is provided with a bag filter device F, and thereby becomes difficult to wash.
FIG. 7 shows another type of apparatus which successively forms granules by supplying a raw-material powder, spraying, in an agitation vessel 300, the powder with a binder liquid, agitating the sprayed powder by agitating blades 301 so as to form circulation flows like vortexes (refer to "Saishin Zoryu-gijutsu no Jissai (actualities of newest granulation technique)" (Sogo Gijutsu Shiryo Shu), published by Kanagawa-ken Keiei Kaihatsu Center (management development center of Kanagawa prefecture), 1984, pp 36-39). This apparatus is also based on the method that utilizes aggregating phenomena due to the moisture in the powder in order to form granules. As described above, it is said that the water content of powder ranges from 5 to 15% in the case of granulation of a powder such as powdered milk where the powder is allowed to have a water content varying over a comparatively wide range, and that it ranges from 5 to 8% in the case of an amino-acid containing material such as soy sauce or a solution of various extracts. However, in an actual apparatus, the progress of granulation becomes slow if the moistness (water content) of the powder is reduced, or the powder attaches to the apparatus or becomes lumps like dumpling if the moistness is high. Thus, the suitable water content range is very narrow. For example, it is several percent in the case of powdered milk, and smaller than 1% in the case of an amino-acid containing material. Therefore, highly improved skill is needed to control the water content.
FIG. 8 shows a so-called Wurster apparatus which is known as a granulation apparatus using a spouted bed granulater with draft tube (refer to "Dai-San-Kai Seizai To Ryushi Sekkei Symposium Koen Yoshi Shu (essentials of lectures in third symposium of formulation and particle design)" published by Ryushi Sekkei Bukai of Funtai Kogaku Kai, Nov. 27, 28, 1986, pp 71-74). In this Wurster apparatus, particles are drawn in by upward flows formed inside a draft tube 400, are blown upward while being sprayed with a binder liquid supplied from a nozzle 401, lose in speed, and start to fall in an expanded section 402 (or is carried on the flows), thus being circulated. Particles accumulated on the bottom are fluidized by air supplied through a perforated plate 403, and are successively drawn into the draft tube 400.
If powder which is prepared by a batch-wise apparatus and which has a wide particle size distribution is processed by this granulation apparatus, there is a possibility of smaller particles segregating, that is, attaching by static electricity to the wall surface or a bag filter 404 disposed in an upper section, or flying to the bug filter 404 by deviating from the circulation flows. This phenomenon reduces the uniformity of granulation.